Storing energy in the form of compressed gas has a long history and components tend to be well tested and reliable, and have long lifetimes. The general principle of compressed-gas or compressed-air energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If a body of gas is at the same temperature as its environment, and expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas will remain at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of high-pressure gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, expansion where no heat is exchanged between the gas and its environment—e.g., because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during expansion and compression, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
An efficient and novel design for storing energy in the form of compressed gas utilizing near isothermal gas compression and expansion has been shown and described in U.S. Pat. No. 7,832,207 (the '207 patent) and U.S. patent application Ser. No. 12/639,703 (the '703 application), the disclosures of which are hereby incorporated herein by reference in their entireties. The '207 patent and the '703 application disclose systems and methods for expanding gas isothermally in staged cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. Mechanical energy from the expanding gas may be used to drive a hydraulic pump/motor subsystem that produces electricity. Systems and methods for hydraulic-pneumatic pressure intensification that may be employed in systems and methods such as those disclosed in the '207 patent and the '703 application are shown and described in U.S. patent application Ser. No. 12/879,595 (the '595 application), the disclosure of which is hereby incorporated herein by reference in its entirety.
In the systems disclosed in the '207 patent and the '703 application, reciprocal mechanical motion is produced during recovery of energy from storage by expansion of gas in the cylinders. This reciprocal motion may be converted to electricity by a variety of means, for example as disclosed in the '595 application as well as in U.S. patent application Ser. No. 12/938,853 (the '853 application), the disclosure of which is hereby incorporated herein by reference in its entirety. The ability of such systems to either store energy (i.e., use energy to compress gas into a storage reservoir) or produce energy (i.e., expand gas from a storage reservoir to release energy) will be apparent to any person reasonably familiar with the principles of electrical and pneumatic machines.
Gas undergoing expansion tends to cool, while gas undergoing compression tends to heat. To maximize efficiency (i.e., the fraction of elastic potential energy in the compressed gas that is converted to work, or vice versa), gas expansion and compression should be as near isothermal (i.e., constant-temperature) as possible. Several techniques of approximating isothermal expansion and compression may be employed.
First, as described in U.S. Pat. No. 7,802,426 (the '426 patent), the disclosure of which is hereby incorporated by reference herein in its entirety, gas undergoing either compression or expansion may be directed, continuously or in installments, through a heat-exchange subsystem external to the cylinder. The heat-exchange subsystem either rejects heat to the environment (to cool gas undergoing compression) or absorbs heat from the environment (to warm gas undergoing expansion). An isothermal process may be approximated via judicious selection of this heat-exchange rate.
Additionally, as described in the '703 application, droplets of a liquid (e.g., water) may be sprayed into a chamber of the cylinder in which gas is presently undergoing compression (or expansion) in order to transfer heat to or from the gas. As the liquid droplets exchange heat with the gas around them, the temperature of the gas is raised or lowered; the temperature of the droplets is also raised or lowered. The liquid is evacuated from the cylinder through a suitable mechanism. The heat-exchange spray droplets may be introduced through a spray head (in, e.g., a vertical cylinder), through a spray rod arranged coaxially with the cylinder piston (in, e.g., a horizontal cylinder), or by any other mechanism that permits formation of a liquid spay within the cylinder. Droplets may be used to either warm gas undergoing expansion or to cool gas undergoing compression. Again, an isothermal process may be approximated via judicious selection of this heat-exchange rate.
An efficient and novel design for the energy-efficient pumping of liquid for the production of liquid sprays used to approximate isothermal expansion and compression inside cylinders or inside other mechanical devices for expanding or compressing gas, as disclosed in the '703 application, has been shown and described in U.S. patent application Ser. No. 13/009,409, filed Jan. 19, 2011 (the '409 application), the entire disclosure of which is incorporated herein. As disclosed in the '409 application, energy-efficient circulation of the heat-exchange liquid through the gas presently undergoing compression or expansion, which during some portion of either compression or expansion is at high pressure (e.g., 3,000 psi), is achieved by circulating the heat-exchange liquid itself at high pressure. This removes any need to raise the pressure of the heat-exchange liquid from atmospheric pressure, which generally increases energy consumption.
During the continuous energy-efficient circulation of a heat-exchange liquid for the purpose of cooling gas undergoing compression as disclosed in the '409 application, the heat-exchange liquid removes heat from the gas and therefore increases in temperature. As the temperature of the heat-exchange liquid increases, the heat-exchange liquid tends to become less capable of removing heat from the gas: if the heat exchange liquid reaches thermal equilibrium with (i.e., becomes the same temperature as) the gas being cooled, heat will cease to be exchanged between the liquid and gas. It is therefore preferable to keep the heat-exchange liquid at a temperature significantly lower than that of the gas undergoing compression.
Similarly, during the continuous energy-efficient circulation of a heat-exchange liquid for the purpose of heating gas undergoing expansion as disclosed in the '409 application, the heat-exchange liquid transfers heat to the gas and therefore becomes cooler. As it cools, the heat-exchange liquid tends to become less capable of transferring heat to the gas: if the heat-exchange liquid reaches thermal equilibrium with the gas being heated, no heat will be exchanged. It is therefore preferable to keep the heat-exchange liquid at a temperature higher than that of the gas undergoing expansion. In other words, there is a need to manage the temperature of the heat-exchange liquid itself, which is at high pressure during at least part of each isothermal compression or expansion cycle.
As detailed above, systems utilizing liquid-spray heat exchange tend to circulate heat-exchange fluid at high pressures (e.g., 3000 pounds per square inch (psi)) through a high-pressure heat exchanger capable of and configured for the circulation of high-pressure liquid. Such high-pressure heat exchangers are generally heavier, larger, more expensive, and more complex than low-pressure heat exchangers of equivalent capacity (i.e., heat-exchange capacity, e.g., in joules/sec) that circulate heat-exchange fluid at significantly lower pressures (e.g., approximately atmospheric pressure). Thus, there is a need for techniques for making low-pressure heat exchange compatible with high-pressure compressed-gas energy storage and recovery.